Certain cross-linked polyurethane hydrogel polymers are known from European Patent Publications EP0016652 and EP0016654. These patent specifications describe cross-linked polyurethanes formed by reacting a polyethylene oxide of equivalent weight greater than 1500 with a polyfunctional isocyanate and a trifunctional compound reactive therewith, such as an alkane triol. The resultant cross-linked polyurethane polymers are water-swellable to form a hydrogel but are water-insoluble and may be loaded with water-soluble pharmaceutically active agents. One particular polyurethane polymer is the reaction product of polyethylene glycol 8000, Desmodur (DMDI i.e. dicyclohexylmethane-4,4-diisocyanate) and 1,2,6-hexane triol and which has been used commercially for vaginal delivery of prostaglandins.
However, such polyurethane polymers possess a number of practical disadvantages. Whilst the use of a triol cross-linking agent is effective in providing polymers of relatively reproducible swelling characteristics, the percent swelling is typically 200-300% (i.e. the increase in weight of the swollen polymer divided by the weight of the dry polymer). Pharmaceutically active agents are loaded by contacting the polymer with an aqueous solution of pharmaceutically active agent, such that the solution becomes absorbed into the polymer, forming a hydrogel. The swollen polymer is then dried back to the chosen water content before use. A consequence is that with the conventional cross-linked polyurethane, the degree of swelling limits the molecular weight of the pharmaceutically active agent which can be absorbed into the hydrogel structure to below about 3000 g/mol. A further disadvantage is that only water-soluble pharmaceutically active agents may be used. Finally, since the conventional cross-linked polyurethane polymer is essentially a non-thermoplastic polymer (thermoset), insoluble in both water and organic solvents, the further processing of the formed polymer into other solid forms, such as films, monolithic devices, foams, wafers, composites, sandwich structures, particles, pellets, foams or coatings, is not possible. In addition, the thermoset nature of the conventional cross-linked polyurethane polymer rules out the possibility of melt mixing drug with the polymer, in order to load the polymer with a suitable active agent without using solvents or water.
Certain thermoplastic polyurethane hydrogel polymers are known from patent Publication WO2004029125. This patent specification describes linear thermoplastic polyurethanes formed by reacting a polyethylene glycol of molecular weight of greater than 4000 g/mol with a polyfunctional isocyanate and a bifunctional compound reactive therewith, such as an alkane diol or diamine. The resultant thermoplastic polyurethane polymers are water-swellable to form a hydrogel but are water-insoluble and may be loaded with water-soluble pharmaceutically active agents. One particular polyurethane polymer is the reaction product of polyethylene glycol 8000, Desmodur (DMDI, i.e., dicyclohexylmethane-4,4-diisocyanate) and 1,10-decane diol, which has shown percentage-swelling from 600% up to 1700% or even above. This type of polymer has shown a suitability for diffusion loading and short-term delivery of relatively water-soluble drugs e.g. clindamycin phosphate, oxytocin, and misoprostol.
However, such a high-swelling thermoplastic polyurethane polymer possesses many practical disadvantages. Due to the high weight content and block length of PEG, the polymer is only suitable for relatively short-term release (i.e. controlled release from 10 min to only a few hours) of active agents, especially in the case of highly water-soluble drugs. In addition, the low hydrophobic content, i.e. low amount of hydrophobic compound e.g. decanediol (DD) or dodecanediol (DDD) makes the polymer inappropriate for hydrophobic drugs and thus restricts its use. Hydrophilic and hydrophobic drugs need to have interactions with both of the phases in order for their release to be controlled by the polymer structure. Further, the imbalance between hydrophobic and hydrophilic compounds hampers microphase separation, which reduces the mechanical strength of the polymer in both the dry and wet state. In addition, due to the high crystallinity of polymer and the formation of hard blocks, the final polymer is rigid and the processing temperature relatively high.
The swelling percentage of high-swelling thermoplastic polyurethanes is typically 200-1700% and is dependent on the PEG content and/or the length of PEG block. Pharmaceutically active agents can be loaded by using the same method as described above for the conventional cross-linked polyurethane, as well as melt mixing drug and polymer. The release time and profiles obtained for the high swelling and crosslinked polyurethane polymers are, however, very similar.
Patent specification WO 94/22934 discloses the production of a linear random block copolymer from polyethylene oxide (number average molecular weight 1000 to 12,000), a diamine and a diisocyanate. Yu et al. Biomaterials 12 (1991) March, No. 2, page 119-120 discloses the use of polyurethane hydrogels formed of polyethylene glycol (number average molecular weight of 5830) and a low molecular weight polypropylene glycol (molecular weight 425) and a diisocyanate. Patent specification U.S. Pat. No. 4,202,880 discloses the production of polyurethanes from polyethylene glycol (molecular weight 400-20,000), an alkaline glycol containing from 2-6 carbon atoms and a diisocyanate. Patent specification U.S. Pat. No. 4,235,988 is a similar disclosure, although the preferred PEG range is 600-6,000.